When simply put on a solid, a liquid drop usually adopts the shape of a spherical cap or a puddle depending on its volume and on the wetting conditions. However, when the drop is subjected to a periodic field, a parametric excitation can induce a transition of shape and can break the drop’s initial axial symmetry, provided that the pinning forces at the contact-line are weak enough. Therefore, a standing wave appears at the drop interface and induces a periodic motion, with a frequency that equals half the excitation frequency. In the first part, we review the different situations where star drops can be generated from various types of periodic excitations. In the second part, we show that similar star drops can occur in a much less intuitive fashion when the drop is put on an air cushion, where no periodic motion is imposed a priori. Preliminary experiments as well as theoretical clues for a hydrodynamic interpretation, suggest that the periodic vibration is due to an inertial instability in the air layer below the drop

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Star-drops formed by periodic excitation and on an air cushion – A short review

We investigate the spontaneous oscillations of drops levitated above an air cushion, eventually inducing a breaking of axisymmetry and the appearance of “star drops”. This is strongly reminiscent of the Leidenfrost stars that are observed for drops floating above a hot substrate. The key advantage of this work is that we inject the airflow at a constant rate below the drop, thus eliminating thermal effects and allowing for a better control of the flow rate. We perform experiments with drops of different viscosities and observe stable states, oscillations, and chimney instabilities. We find that for a given drop size the instability appears above a critical flow rate, where the latter is largest for small drops. All these observations are reproduced by numerical simulations, where we treat the drop using potential flow and the gas as a viscous lubrication layer. Qualitatively, the onset of instability agrees with the experimental results, although the typical flow rates are too large by a factor 10. Our results demonstrate that thermal effects are not important for the formation of star drops and strongly suggest a purely hydrodynamic mechanism for the formation of Leidenfrost stars

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Oscillating and star-shaped drops levitated by an airflow.

Droplet oscillation and pattern formation during Leidenfrost phenomenon

We report an experimental study of a magnetic liquid drop deposited on a superhydrophobic substrate and subjected to vertical vibrations in presence of a static magnetic field. It is well-known that a flattened drop of usual liquid displays oscillating lobes at its periphery when vibrated. By adding ferromagnetic nanoparticles to a water drop and varying the strength of the magnetic field, we are experimentally able to efficiently tune the resonant frequencies of the drop. By using conservation energy arguments, we show that the magnetic field contribution is equivalent to adding an effective negative surface tension to the drop. Our model is found in good agreement with the experiments with no fitting parameter.

https://hal.archives-ouvertes.fr/hal-01306787/document

Tuning the resonant frequencies of a drop by a magnetic field

Effect of the magnetic field and current orientation on the splashing of liquid metal free surface of fusion reactor PFCs

New types of electromagnetic parametric instability have been observed at the periphery of the free surface of a liquid metal pool in the presence of a low-frequency magnetic field. An experimental set-up is used to observe the motion of a mercury layer on a substrate located in a solenoidal coil supplied with low-frequency alternating electric currents. The Lorentz body forces produced are mainly oscillatory and generate motion in the liquid. Various regular and irregular free-surface patterns are observed. Two-dimensional stability analysis shows that axisymmetric waves are directly forced by the electromagnetic forces while azimuthal waves are the result of instability. The experimental stability diagram exhibits 'tongues' as already observed for parametric resonance instability. For high magnetic field strengths, the free-surface patterns become highly unstructured. High-amplitude 'fingers' as well as solitary waves are observed. Measurements of the deformation observed on photographs are compared with rough theoretical estimates

Y. Fautrelle

Papers

Free-surface horizontal waves generated by low-frequency alternating magnetic fields